phase align circuit

[featherpillow]

I searched the board for something similar to this, and couldn't find one. I thought a few people might find this useful.

Here's a simple JFET phase alignment circuit. It's not anything fancy, it gives a full 360 degrees of phase rotation, which might be a bit of overkill for the studio. If you only want 180 degrees of phase rotation, eliminate Q2 and associated components. It uses a dual ganged 1M pot for R5, which should be changed to a single 1M pot for a 180 degree version.

Idss is about 3ma for the first stage, and 1.5ma for the second, given a 15 volt supply. The FETs I used for this are 2N4416's.

 

[bcarso]

Note well, though, that this phase shift is at a particular frequency...

 

[featherpillow]

Note well, though, that this phase shift is at a particular frequency...

Governed by the value of C1 and C2?

 

[bcarso]

They set the range along with other values. This is a version of the 3R 1C allpass using an op amp, with two stages in series.

At the extremes the phase shift is broadband, but as one would expect from an inverter. The freq sensitivity is probably maximum at the 90 degree region. Sims would be instructive, or a little circuit math.

To get some serious broadband shifts you need to chain a fair number of stagger-tuned stages. There are some published circuits somewhere for getting a net audio-band or at least wide speech-band 90 degree phase difference outputs for use in single-sideband modulators.

There were also some RLC filter boxes that did this and are probably still floating around (no pun intended) out there. My friend Shelton called these Hilbert transformers.

Put down that jeweler's saw CJ!!!

 

[clintrubber]

that this phase shift is at a particular frequency...

Just build a few more for the other frequencies ! :wink: :thumb: :green:

But serious, there could be some more about this here, or was it again at the old place...
There were scans, and discussion of the LittleLab-box.
But while it may work faster or sounding more smooth, I haven't understood the difference yet w.r.t. moving tracks around a bit in time (if you're using a DAW - and it's assuming you want same shift in time for all signals).

 

[featherpillow]

My friend Shelton called these Hilbert transformers

I've read a little about the Hilbert transformer. You can place several phase-shift circuits in parallel with each other, varying the R/C values to cover the bandwidth of the audio range.

I haven't understood the difference yet w.r.t. moving tracks around a bit in time

I was under the impression that it helps make up for phase-related mistakes made during tracking. I guess, though, that raises the question "why not just re-track it...?"

 

[soundguy]

well, there's some stuff thats unavoidable, like the lag between a DI bass if it is taken in series with the bass cab, there's always a bit of delay there that is inherent in the design. A lot of the time also, you can get some wierd stuff with a snare which at normal mix volumes isnt incredibly noticable or destructive but when you slam the limiter for the bridge part, adding 15dB of makeup gain really makes the phase shift noticable and does more harm than good and for such a little part, hard to justify retracking that far down the line. I have an IBP and use it all the time, there's always some little thing that seems to pop up. Id say if you are using 8 of them on a mix, well...

dave

 

[clintrubber]

"why not just re-track it...?"

As soundguy said, there'll be differences that no re-tracking can solve.
But shifting a certain track (say shifting a DI'd track) w.r.t. the miked version is simple in a DAW, yet seems better (perhaps just faster) working with that LittleLabs-box, which is still puzzling me.

 

[featherpillow]

Well that certainly makes sense--I didn't realize that there was the potential for that much phase lag throughout the signal chain (although now that its been pointed out to me it seems rather obvious).

I tried searching the archives for something about the IBP, but still couldn't find anything.

It's time to do some more research on Hilbert Transformer, and expand that circuit a little more.

edit: I found some IBP threads. Apparently, PRR says the circuit is theoretically simple, but in practice its nothing to sneeze at.

 

[clintrubber]

FWIW, here's some more about the IBP:
http://www.soundonsound.com/sos/nov03/articles/littlelabs.htm

 

[Guest]

[quote author="featherpillow"] I thought a few people might find this useful.
[/quote]
Simple cascaded first order allpass.
Not enough for OPTIMOD.
I have somewhere here posts about second order allpasses.
where Q can be modified for higher than one.


xvlk